1. Field of the Invention
This invention relates to gas turbine engine apparatus for minimizing engine case deflection under gust and thrust loads and particularly to means for controlling the engine backbone bending.
The concepts were developed specifically for high bypass ratio, ducted fan engines, but have broader applicability to low bypass ratio and turbojet engines as well.
1. Description of Related Art
A principal type of modern aircraft gas turbine engines in usage today are of the turbofan type. A portion of the working medium gases is directed from the engine inlet through the compression, combustion, and turbine sections of the engine core. The remaining portion of the working medium gases is directed through the fan section, around the engine core, and discharged directly to the atmosphere to produce thrust. The diameter of the engine at the fan stages is typically significantly larger than the core engine diameter, on the order of two to one and larger for high bypass ratio engines of the eighty thousand pound thrust class.
Each engine is supported by an aircraft structure, for example, on a pylon extending downwardly beneath the wing. The engine is typically mounted and secured to the aircraft in two planes normal to the engine centerline, one towards the forward end of the engine, usually just rearward of the fan section and a second toward the aft end of the engine, typically in the turbine section. The engine is mounted by its static structure which supports the rotating components generally referred to as rotors. The engine static structure generally has sub-structures including a forward frame and a aft frame connected by an engine casing often referred to as a backbone. Forward and aft frames having radially extending structural struts which typically support the engine bearings which in turn rotatably support the rotors. Typically a dual rotor engine has a forward fan frame and a rear turbine frame that support the main rotor bearings wherein the fan frame supports a thrust bearing and the rear turbine supports a roller bearing.
Maintaining the engine backbone concentrically about the engine rotor is of obvious criticality, and a constant objective of gas turbine scientists and engineers. Prior art generally disclose case stiffening methods to reinforce the frame. Among the ideas disclosed in the prior art is using the annular cowl structure to reinforce or generally add support to the engine backbone. Many of these ideas add weight and cost to the engine and are not very effective when used with today's low weight metallic or composite cowls. Furthermore the prior art designs are not capable or have limited capability to respond to varying flight conditions which cause correspondingly varying degrees of backbone deflection.
Backbone deflection due to axial and vertical loads is additive because both loads cause a deflected shape wherein the engine casing or backbone structure between the forward and aft mounts is deflected downward relative to the undisturbed engine centerline. High engine power aircraft climb can cause predictable yet varying backbone deflection while wind gusts acting on the nacelle may cause unpredictable and varying backbone deflection which is additive.
To maximize gas turbine engine performance and minimize specific fuel consumption it is desirable to have the engine static structure remains straight and parallel to the engine rotor centerline under all operating conditions. Typically this is achieved through proper design of the engine static structure, engine mounts and engine rotors. It is highly desirable to maintain close clearances between rotating and static elements such between the engine rotor blades and engine casings because this clearance has a significant effect on engine performance and specific fuel consumption. An increase in turbine blade tip clearance allows more of the working fluid to bypass the turbine blades without useful work being extracted. Compressor efficiency is reduced in the compressor section as compressor blade tip clearance increases. Overall engine cyclic efficiency is reduced as labyrinth seal clearances increase. Bending of the engine static structure increases these clearances through abrasive wear between the engine rotating hardware and the engine casing supported stationary hardware.
As the thrust load developed by modern turbofan engines has increased, so has the magnitude of the reaction loads and bending moment. The resultant engine static structure deflection causes increased rubbing between the rotating elements and the adjacent stationary elements. This abrasive wear results in an adverse impact on engine performance and specific fuel consumption, and necessitates more frequent engine maintenance and overhaul. Repair and replacement of rotor blades is one of the highest operating costs for an aircraft gas turbine engine.
Increasing fuel costs and demands for improved durability accentuate the need for low weight designs and systems for minimizing backbone deflection from the engine centerline axis under varying engine operating conditions.